In this setup, anytime the dispenser gets a siganal, something happens: (sand is dispensed, bone meal and water buckets just disappear).
Unfortunately, the plant (I tested with cotton) disappears immediately, even in case of an empty dispenser.
Plus, somehow the dispenser only works while a pipe is connected to the pipe-input-slot.

My Question:
Did i do the setup wrong or is it ( yet ) impossible to get plants fertilized by dispenser?

I think I had trouble with getting seeds to dispense on the soil, and had to place the dispenser under the ground node to get the seeds to actually grow. It might be possible for the bonemeal to work if it is in a dispenser placed underground and pointing up towards the dirt above it.